Current methods for obtaining "body-fit" features in personal care products use mechanical fasteners, woven elastic band structures, elastic nonwoven laminates, or glued-in elastic strands. All have drawbacks to some degree when measured against the three criteria of cost, performance, and aesthetics. With respect to the elastic components, development of elastic nonwoven laminates (e.g., waist elastic, stretchable side panels, Lycra.RTM. strand laminates) has been leveraged in products to give body fit innovations with aspects of cloth-like aesthetics. These stretchable structures are fabricated in a "flat" or planar geometry. This form suits existing base sheet and product assembly technologies; however, it introduces complexities that require sophisticated solutions, especially in the converting process. The invention when used in the form of a seamless band or tubular structure provides an alternative to such flat structures.
Bicomponent filaments in a side-by-side configuration are defined as having a "conjugate" arrangement. Almost all synthetic conjugate fibers have self-crimp potential. The crimp, helical in structure, usually manifests itself in melt-spun filaments after they are subjected to a post-treatment that induces shrinkage in the components. (Commonly used treatments are heat, moisture, and neck-stretching.) The crimp-forming potential of conjugate fibers is primarily related to the difference in shrinkage characteristics of the individual components. The shrinkage results from internal structural changes that are triggered by temperature- and/or time-dependent phase changes (crystallization factors being most prevalent).
Processing conditions will not produce helical crimping without a shrinkage differential between the components. Even the crimp resulting from asymmetric quenching of polypropylene is due to a conjugate arrangement of different crystalline structures. However, they do impact the extent of crimp development. Because most self-crimping forces are low, they are usually overpowered by attenuation forces. As a result, most spun conjugate filaments exhibit no crimp. For certain component combinations, spinning conditions can be found that result in spontaneous crimping (once the drawing forces are relaxed) without the need of a post-treatment.
Crimp in a fiber causes greater bulk in fabric form, it changes the tactile properties (e.g., drape and feel), and it has the potential for imparting the additional feature of stretch. This is the case for both mechanically induced-crimped and self-crimped filaments. In self-crimped filaments the ability to stretch arises from their helical, spring-like structure, which is geometrically distinct from the "saw tooth" structure of mechanically crimped filaments. The stretch consists of both extension and recovery aspects. In extension, the crimped fiber shows a nonlinear, low stress response as the crimp geometry deforms, then a high stress response as the fiber is completely extended. Recovery, if it occurs after extension, is by crimp "regain."
Because their recovery is linked to crimp regain (a physical manifestation of relatively low internal forces) most conventional self-crimping fibers lack the power retraction of Lycra.RTM. and other purely elastic fibers. The power retraction of elastomers are a consequence of their molecular structure. Lycra.RTM.-like filaments (from dry-spun polyurethane), rubber strands, and thermoplastic elastomers (e.g. Kraton.RTM. polymers, Arnitel.RTM. polymers, melt-spun polyurethanes) are all segmented block copolymers. The elastic properties arise from alternating molecular sequences of soft chain segments bonded together with hard or rigid chain segments. In a relaxed state the soft chains lie in a tangled disorder; under tension the chains straighten out while always straining back to their natural tangle. While elastomeric fibers develop an immediate molecular resistance under tension, no such resistance occurs for crimped fibers until the crimp is pulled out and cold-drawing deformation begins.
Polyurethane-based fibers attenuated from the melt, as disclosed in the prior art, do not exhibit spontaneous elastomeric properties (recovery after stretch). Rather, these fibers must be aged for a period of time, some up to approximately twenty four hours, which increases significantly the cost and time to produce product. Additionally, post-formation treatment, e.g., stretching, is normally required. Polyurethane filaments are not known to crimp when attenuated from melt. See, for example, U.S. Pat. Nos. 3,379,811; 4,551,518; and 4,660,228.
U.S. Pat. No. 3,761,348, issued to Chamberlin, discloses a helically crimped biconjugate filament composed of a polyester and an elastomeric polyurethane. Once the filaments are formed (spun) they are aged and only then stretched via a post-spinning step to develop crimps. The required aging and post-spinning stretching step introduces additional time and expense into the manufacturing process.
U.S. Pat. No. 4,405,686, issued to Kuroda et al., discloses a highly stretchable crimped elastic filament resulting from the biconjugate combination of an elastomer and a non-elastomer having specified cross-sectional shapes (e.g., bilobal). The stretch capabilities of the filaments in the filament are described as having two states: a low elongation state where the stretch due to crimp is dominant and a high elongation state where the stretch due to the elastomer is dominant. As in Chamberlin, the spun filaments must be drawn in a subsequent step in order to develop the crimp that dominates the stretch characteristics at low elongations. Again, this separation of steps increases expense and time to produce product.
There is a need then, for a fiber composition that will produce self-crimping fibers absent post-treatment steps. Such a fiber would have high extensibility while exhibiting high recovery properties. Such a fiber could be used to impart form-fitting (body conforming) attributes to incontinent garments (e.g., diapers), hospital garments (gowns), bandages and body wraps as well as personal garments, where compressive force is needed, as well as in personal garments, such as underwear and the like.
It is a principal object of the present invention to provide melt attenuated conjugate filaments having improved crimping and extensibility properties without the need of a post-stretching or tensioning step.
It is a further object of the present invention to provide a method of forming melt attenuated conjugate filaments which can be immediately wrapped after melt attenuation to form a band having improved extensibility in the radial direction and a high degree of recovery.
Other objects, features, and advantages of the present invention will become apparent upon reading the following detailed description of embodiments of the invention, when taken in conjunction with the accompanying drawing and the appended claims.